BUGFIX: Proper handling of blocks with 0 particles
BUGFIX: Proper handling of blocks with 0 particles

The code was asserting that each block has particles
which may not always be the case. This commit fixes
the issue by creating and returning an empty
vtkPolyData.

keithroe/vtkoptix,keithroe/vtkoptix,keithroe/vtkoptix,aashish24/VTK-old,hendradarwin/VTK,sumedhasingla/VTK,ashray/VTK-EVM,hendradarwin/VTK,biddisco/VTK,SimVascular/VTK,berendkleinhaneveld/VTK,mspark93/VTK,candy7393/VTK,mspark93/VTK,ashray/VTK-EVM,collects/VTK,berendkleinhaneveld/VTK,SimVascular/VTK,candy7393/VTK,demarle/VTK,gram526/VTK,sumedhasingla/VTK,keithroe/vtkoptix,msmolens/VTK,aashish24/VTK-old,collects/VTK,berendkleinhaneveld/VTK,ashray/VTK-EVM,berendkleinhaneveld/VTK,jmerkow/VTK,sankhesh/VTK,demarle/VTK,sankhesh/VTK,johnkit/vtk-dev,msmolens/VTK,gram526/VTK,collects/VTK,demarle/VTK,mspark93/VTK,hendradarwin/VTK,ashray/VTK-EVM,mspark93/VTK,mspark93/VTK,sumedhasingla/VTK,keithroe/vtkoptix,jmerkow/VTK,ashray/VTK-EVM,hendradarwin/VTK,candy7393/VTK,ashray/VTK-EVM,candy7393/VTK,gram526/VTK,SimVascular/VTK,ashray/VTK-EVM,sankhesh/VTK,msmolens/VTK,jmerkow/VTK,gram526/VTK,candy7393/VTK,biddisco/VTK,berendkleinhaneveld/VTK,collects/VTK,demarle/VTK,aashish24/VTK-old,aashish24/VTK-old,keithroe/vtkoptix,candy7393/VTK,cjh1/VTK,cjh1/VTK,sankhesh/VTK,collects/VTK,johnkit/vtk-dev,SimVascular/VTK,gram526/VTK,sumedhasingla/VTK,gram526/VTK,demarle/VTK,biddisco/VTK,mspark93/VTK,keithroe/vtkoptix,johnkit/vtk-dev,SimVascular/VTK,jmerkow/VTK,aashish24/VTK-old,msmolens/VTK,hendradarwin/VTK,mspark93/VTK,biddisco/VTK,jmerkow/VTK,gram526/VTK,cjh1/VTK,jmerkow/VTK,aashish24/VTK-old,msmolens/VTK,sumedhasingla/VTK,cjh1/VTK,sankhesh/VTK,demarle/VTK,sumedhasingla/VTK,SimVascular/VTK,collects/VTK,candy7393/VTK,ashray/VTK-EVM,johnkit/vtk-dev,keithroe/vtkoptix,sumedhasingla/VTK,johnkit/vtk-dev,msmolens/VTK,msmolens/VTK,gram526/VTK,demarle/VTK,SimVascular/VTK,cjh1/VTK,demarle/VTK,sankhesh/VTK,sumedhasingla/VTK,biddisco/VTK,berendkleinhaneveld/VTK,cjh1/VTK,sankhesh/VTK,johnkit/vtk-dev,jmerkow/VTK,hendradarwin/VTK,johnkit/vtk-dev,jmerkow/VTK,sankhesh/VTK,biddisco/VTK,biddisco/VTK,mspark93/VTK,hendradarwin/VTK,berendkleinhaneveld/VTK,msmolens/VTK,candy7393/VTK,SimVascular/VTK